As a method for transmitting a signal at high speed in digital radio communications, it is common to use (1) a method of increasing a symbol rate or (2) a method of performing multilevel modulation.
However, in the case (1) of increasing the symbol rate, it is necessary to increase a speed of digital circuits such as an A/D converter and a D/A converter or broaden a band of a baseband circuit, which raises a problem that those circuits may be hard to realize or their power consumption may considerably increase.
Meanwhile, in the latter case (2) of performing the multilevel modulation, a load on the digital circuit or the baseband circuit is reduced, but an envelope of a modulated signal fluctuates, thereby increasing a ratio of a peak power to an average power of the signal (peak to average power ratio (PAPR)). To linearly amplify such a signal having a large PAPR, it is necessary to use an amplifier having a large saturation power, and further necessary to operate the amplifier with a back-off at a sufficient level from a saturation power, but there is a problem that the amplifier operated with a large back-off exhibits a considerably low efficiency.
In particular, in a communication system using a microwave or millimeter wave range in which an amplifier having a high power and a high efficiency is hard to realize, a transmission device is high in cost and considerably low in efficiency, which results in a problem that an expensive heat radiator becomes necessary or other such problem.
Further, for another scheme for performing high-speed transmission by radio communications, there is a method of using multiple carriers. FIG. 13 is a block diagram illustrating a configuration of a conventional radio transmission device which uses multiple carriers (see, for example, Patent Document 1).
In the conventional radio transmission device illustrated in FIG. 13, an input digital signal is input in series to a data input section 101, the serial input digital signal input to the data input section 101 is subjected to serial-to-parallel conversion into a plurality of channels by a series/parallel converter 102, and the converted digital signal in each of the channels is encoded by a convolutional encoder 103. The convolutionally encoded digital signal is multilevel-modulated by a modulator 104. The multilevel-modulated signals are converted into a plurality of signals having different carrier frequencies by frequency converters 105a to 105d, then the plurality of signals are synthesized by a multiplexer 106, and after that, the resultant is radio-transmitted from an antenna 107.
In such a conventional radio transmission device, since the carrier frequencies of the radio-transmitted signals are discrete, the signals adjacent in frequency are unlikely to simultaneously disappear due to fading, and since the convolutionally encoded signals are used, even if a part of signals disappear, the part can be restored, which makes it possible to realize a radio device which exhibits satisfactory communication quality even under a fading environment. However, there occurs a problem that a frequency bandwidth of the channels as a whole becomes broader.
Further, although not particularly illustrated in the block diagram of the conventional radio transmission device of FIG. 13, in a case of structuring and configuring the device in actuality, it becomes possibly necessary to use an amplifier to obtain an electric power required to perform radio transmission. In this case, as illustrated in FIG. 14, one amplifier 108 may be inserted between the multiplexer 106 and the antenna 107. However, in the case of FIG. 14, in order to amplify the multiplexed signals, signals having a large PAPR are input to the amplifier 108. Therefore, it is necessary to use the amplifier 108 having a large saturation power, and the amplifier 108 is also used with a back-off from a saturation power level, which raises a problem that the efficiency of the amplifier 108 becomes lower. In a case of a multicarrier scheme using a fast Fourier transfer (FFT) and an inverse FFT (IFFT), a circuit configuration of FIG. 14 is employed, which leads to an extremely large problem.    Patent Document 1: JP 3346945 B